The objective of this study is to investigate the microbial community and its characteristics changes aroused by the ratios of COD/SO42- in the acidogenic-phase reactor of two-phase anaerobic process. A continuous-flow lab-scale test was conducted in an acidogenic sulfate-reducing reactor with molasses wastewater as sole organic carbon source and sodium sulfate as the electron acceptor. The experimental results showed that Acetic-acid Type Microbial Metabolism resulted in the formation of An Acetic-acid Type Climax Community. The change of the COD/SO42- ratio caused an ecological succession from a stable climax community at moderate and high COD/SO42- ratios to a sub-stable climax community at a lower COD/SO42- ratio. But Acetic-acid Type Microbial Metabolism kept unchanged during this course, which indicated the stability of Acetic-acid Type Climax Community. The Acetic-acid Type Microbial Metabolism and Acetic-acid Type Climax Community were of typical characteristics in the acidogenic sulfate-reducing reactor.